This section provides background information related to the present disclosure which is not necessarily prior art.
Multilayer magnetoelectronic devices rely on the property of particular multilayer structures, which include ferromagnetic layers separated by intermediate nonmagnetic layers, that their electric resistance changes upon application of an external magnetic field. More particularly, this so-called magnetoresistance is due to the fact that the electric resistance depends on the relative orientation of the magnetization of the individual ferromagnetic layers, and that an external magnetic field may change this relative orientation. Two effects resulting in magnetoresistance are the so-called giant magnetoresistance (GMR) effect, which may occur in multilayer structures in which the intermediate nonmagnetic layers are electric conductors, and the tunnel magnetoresistance (TMR) effect, which may occur in multilayer structures in which the intermediate nonmagnetic layers are electric insulators.
In order to be useful in practice, magnetoresistive multilayer structures must be manufactured with a defined relative orientation of the magnetization of the individual layers, i.e. of their magnetic moments which are vectorial quantities. For this reason, many of the known magnetoresistive multilayer structures are rather complex and difficult to prepare. In prior art magnetoresistive multilayer structures the orientation of the magnetization of adjacent ferromagnetic layers differs by 0° or 180°. Further, in laboratory experiments multilayer structures in which the orientation of the magnetization of adjacent ferromagnetic layers differs by 90° have been created, but turned out to be very difficult to produce reliably and have not be utilized in actual magnetoelectronic devices.
For example, in one type of magnetoresistive multilayer structure the materials and thicknesses of the ferromagnetic layers and the intermediate nonmagnetic layers are carefully chosen and controlled such that adjacent ferromagnetic layers are coupled by the so-called RKKY interaction, which effects a relative orientation of the magnetic moments of the respective two ferromagnetic layers. The RKKY interaction varies with, amongst others, the thickness of the intermediate nonmagnetic layer, so that the orientation of the magnetization of adjacent ferromagnetic layers can be chosen to be either parallel or antiparallel in remanence (the above-mentioned relative orientations of 90° are very difficult to realize in a stable manner). A suitable external magnetic field is able to overcome the coupling strength of the RKKY interaction and to change the direction of the magnetization of at least some of the ferromagnetic layers, thereby changing the electric resistance of the multilayer structure. For example, the orientation may be forced to change from antiparallel in remanence to parallel upon application of the external magnetic field. The resulting change of the magnetoresistance of the multilayer structure can be used to detect the presence of the external magnetic field.
Due to the fact that in practice it is only possible reliably to realize parallel or antiparallel relative orientations and that the strength of the interlayer coupling provided by the RKKY interaction, which strength influences the saturation behavior of the coupled ferromagnetic layers in an extremely sensitive manner and changes upon varying the thickness of the intermediate layer in the order of 0.1 nm, the possibilities to selectively precisely set the switching field and the general magnetic field dependence of the magnetoresistance are very limited. Consequently, the design of corresponding magnetic field sensors adapted to particular applications is restricted. Further, the RKKY interaction is only observed for a very limited number of material combinations for the ferromagnetic layers and the intermediate layers and for very thin intermediate layers.
A further example for magnetoresistive multilayer structures are so-called exchange-biased spin valves, in which an antiferromagnetic layer is arranged in contact with one ferromagnetic layer in order to fix or “pin” the magnetization of that ferromagnetic layer by means of an exchange interaction. A further ferromagnetic layer is separated and decoupled from the fixed ferromagnetic layer by a nonmagnetic intermediate layer. Consequently, this “free” ferromagnetic layer may change its magnetization upon application of a suitable external magnetic field while maintaining the magnetization of the fixed layer. Spin valve systems can be classified depending on the electric properties of the nonmagnetic intermediate layer. So-called GMR type spin valves utilize electrically conductive intermediate layers, and so-called TMR (tunnel magnetoresistance) type spin valves utilize electrically insulating intermediate layers, which act as tunnel barriers. TMR spin valves are used, for example, in magnetoresistive random access memory (MRAM) devices. In order to reduce or avoid stray fields generated by the pinned layer and an undesired coupling between the pinned layer and the free layer, a combination of two antiparallel ferromagnetic layers may be used as pinned layer instead of a single pinned ferromagnetic layer. Such a pinned combined layer may be arranged such that its overall magnetic field is zero or near zero at the free layer. This construction of the pinned layer is commonly utilized in, for example, MRAM devices.
The antiferromagnetic layer in these spin valves is part of a rather complex pinning structure, which complicates the manufacturing of such spin valve structures. Additionally, also in this case the possibilities to selectively precisely set the switching field and the general magnetic field dependence of the magnetoresistance are very limited, because the switching behavior of the free magnetic layer can hardly be influenced and no easy magnetic axis, i.e. no preferred direction for the magnetic moment, is typically induced in the free layer.
In any case, it is problematic to reliably prepare ferromagnetic layers having a defined magnetic anisotropy in the multilayer structures. In this regard, only the above-mentioned relative orientations of 0° and 180° (and 90° in laboratory experiments) were realized.
From U.S. Pat. No. 6,818,961 it is known to prepare the ferromagnetic layers of magnetoresistive multilayer structures by oblique incidence deposition, wherein depending on the angle between the incident particles being deposited and the direction perpendicular to the plane of extension of the ferromagnetic layers the easy axis of the resulting uniaxial anisotropy is either parallel or perpendicular to the plane defined by the direction of incidence of the particles and the projection of that direction onto the plane of extension of the ferromagnetic layers. According to this prior art document, smaller angles result in a parallel orientation of the easy axis and larger angles result in a perpendicular orientation of the easy axis, and by varying the angle of incidence it is furthermore possible to vary the magnetic hardness of the layers. The technique was used to prepare multilayer structures having parallel relative orientations of the magnetization of the ferromagnetic layers.
Irrespective of the type of magnetoresistive multilayer structure used known methods of producing multilayer magnetoelectronic devices provide only a limited capability to adapt them to a particular application. This also entails that for some applications multilayer magnetoelectronic devices are merely of limited usefulness or cannot be used altogether. For example, if at all possible, it is very complicated to provide a multilayer magnetoelectronic device having a defined and desired qualitative or quantitative sensitivity for a particular application.